Experimental and numerical investigation of structures of two-dimensional partially premixed methane/air flames

Abstract

Studies on laminar partially premixed flames are essential for the design of efficient household boilers. An idealized boiler has been designed recently to facilitate experimental and numerical investigations using various non-intrusive measurement techniques and two-dimensional computational fluid dynamics (CFD) codes. This boiler mimics a commercial one in terms of power density and performance. Three test cases were selected for the present study to assess the accuracy of predictions made using a recently developed CFD code. The global equivalence ratio (primary air+secondary air) for all three cases was the same: however, the local equivalence ratio of the fuel was varied to obtain different partially premixed flame configurations. While two cases resulted in Bunsen-type flames, the third one yielded a V-shaped flame. Spontaneous emissions from CH and OH radicals in these partially premixed flames were measured using charge-coupled-device (CCD) cameras, and the results were compared with the predictions. Calculations were performed using 24-species modified Peters chemical kinetics and a 31-species GRI-V1.2 mechanism. The weak inner premixed and outer diffusion flames in case 2, the Bunsen-type flame in case 4, and the V-type flame in case 5 were accurately simulated by the modified Peters mechanism. The predicted flame shapes and heights matched well with those obtained in the experiments. On the other hand, even though GRI-V1.2 chemistry seems to predict stable flames well, it is found to fail in predicting flame structures for the conditions that are near the blowout limits. This observation on the GRI mechanism in simulating partially premixed flames is consistent with the longer standoff distances found in our earlier studies on the diffusion flame base.